U.S. patent number 6,151,966 [Application Number 09/304,849] was granted by the patent office on 2000-11-28 for semiconductor dynamical quantity sensor device having electrodes in rahmen structure.
This patent grant is currently assigned to Denso Corporation. Invention is credited to Kenichi Ao, Seiji Fujino, Tsuyoshi Fukada, Hirofumi Higuchi, Kazuhiko Kano, Minoru Murata, Hiroshi Muto, Minekazu Sakai, Hiroshige Sugito, Yukihiro Takeuchi.
United States Patent |
6,151,966 |
Sakai , et al. |
November 28, 2000 |
Semiconductor dynamical quantity sensor device having electrodes in
Rahmen structure
Abstract
A semiconductor accelerometer device is formed on an SOI
substrate by micro-machining. A movable unit is supported at both
ends, and a weight portion is movable in response to acceleration
exerted in the detection direction. A movable electrode is formed
in a comb shape integrally with the weight portion. A pair of fixed
electrodes in a comb shape are cantilevered and interleaved with
the movable electrode to face the movable electrode. A plurality of
through holes is provided in the electrodes so that the electrodes
have Rahmen structure which is a series of rectangular frames. This
structure reduces the weight of each electrode while increasing the
strength against twist force. The electrodes are less likely from
breaking in response to an acceleration exerted in a direction
perpendicular to the normal detection direction because of reduced
weight.
Inventors: |
Sakai; Minekazu (Kariya,
JP), Takeuchi; Yukihiro (Nishikamo-gun,
JP), Kano; Kazuhiko (Toyoake, JP), Fujino;
Seiji (Toyota, JP), Fukada; Tsuyoshi (Aichi-gun,
JP), Sugito; Hiroshige (Nagoya, JP),
Murata; Minoru (Kariya, JP), Muto; Hiroshi
(Nagoya, JP), Higuchi; Hirofumi (Okazaki,
JP), Ao; Kenichi (Tokai, JP) |
Assignee: |
Denso Corporation (Kariya,
JP)
|
Family
ID: |
14959506 |
Appl.
No.: |
09/304,849 |
Filed: |
May 5, 1999 |
Foreign Application Priority Data
|
|
|
|
|
May 11, 1998 [JP] |
|
|
10-127419 |
|
Current U.S.
Class: |
73/514.32;
73/862 |
Current CPC
Class: |
G01P
15/0802 (20130101); G01P 15/125 (20130101); G01P
2015/0814 (20130101) |
Current International
Class: |
G01P
15/125 (20060101); G01P 15/08 (20060101); G01P
015/125 (); G01L 001/00 () |
Field of
Search: |
;73/514.32,514.36,514.38,862.52,862.541,862.68,862,862.381 |
References Cited
[Referenced By]
U.S. Patent Documents
Primary Examiner: Moller; Richard A.
Attorney, Agent or Firm: Pillsbury Madison & Sutro
LLP
Parent Case Text
CROSS REFERENCE TO RELATED APPLICATION
This application relates to and incorporates herein by reference
Japanese Patent Application No. 10-127419 filed on May 11, 1998.
Claims
We claim:
1. A semiconductor sensor device comprising:
a semiconductor substrate;
a movable unit supported on the semiconductor substrate, and having
a weight portion movable in response to a dynamical force exerted
thereon and a movable electrode formed integrally with the weight
portion and having a detection surface; and
a fixed electrode cantilevered on the semiconductor substrate and
having a detection surface facing the detection surface of the
movable electrode,
wherein the movable electrode and the fixed electrode are
constructed in Rahmen structure which includes a series of a
plurality of rectangular frames.
2. A semiconductor sensor device of claim 1, wherein:
the weight portion is constructed in the Rahmen structure.
3. A semiconductor sensor device of claim 1, wherein:
the movable unit further has an anchor which connects the weight
portion to the semiconductor substrate and is constructed in the
Rahmen structure.
4. A semiconductor sensor device of claim 1, wherein:
the movable electrode is connected to the weight portion at a
position where to rectangular frames of the weight portion are
connected.
5. A semiconductor sensor device of claim 1, further
comprising:
a protrusion formed on at least one of the movable electrode and
the fixed electrode to restrict sticking between the movable
electrode and the fixed electrode, the protrusion being provided at
a position where the rectangular frames are connected.
6. A semiconductor sensor device of claim 1, further
comprising:
a movable part having a spring which supports the weight portion,
wherein the spring has a protrusion at a position where rectangular
frames of the weight portion are connected.
7. A semiconductor sensor device of claim 1, wherein:
the movable electrode and the fixed electrode have tapered
parts.
8. A semiconductor sensor device of claim 1, further
comprising:
pads formed on the semiconductor substrate and connected
electrically to the movable electrode and the fixed electrode, the
pads being surrounded by a plurality of grooves to be insulated
electrically from surrounding parts.
9. A semiconductor sensor device of claim 8, further
comprising:
a bank wall provided between the grooves, wherein the bank wall is
cut dynamically and electrically at least at one location.
10. A semiconductor sensor device of claim 1, wherein:
the rectangular frames of the Rahmen structure of the weight
portion and the fixed electrode has a uniform wall thickness or a
uniform spacing between walls.
11. A semiconductor sensor device of claim 10, wherein:
the weight portion has a wall thickness different from the uniform
wall thickness at a position where the movable electrode is
connected.
12. A semiconductor sensor device of claim 10, wherein
a first spacing exists between the fixed electrode and the
detection surface of the movable electrode and a second spacing
exists between the fixed electrode and a non-detection surface of
the movable electrode, the first spacing and the second spacing
being different from each other.
13. A semiconductor sensor device of claim 10, wherein
a first spacing exists between the fixed electrode and the weight
portion and a second spacing exists between the fixed electrode and
a non-detection surface of the movable electrode, the first spacing
and the second spacing being equal to each other.
14. A semiconductor sensor device of claim 11, wherein:
the wall thickness of the weight portion is equal to a total width
of each rectangular frame of the movable electrode and the fixed
electrode.
15. A semiconductor sensor device of claim 10, wherein:
a total width of each rectangular frame of the movable electrode
equals a total width of each rectangular frame of the fixed
electrode.
16. A semiconductor sensor device of claim 1, wherein:
at least one of the movable electrode and the fixed electrode has
protrusions on connection parts.
17. A semiconductor sensor device of claim 6, wherein:
the spring has reinforcing parts.
18. A semiconductor sensor device of claim 1, wherein:
the semiconductor substrate is an SOI substrate which has a first
semiconductor layer, a second semiconductor layer and an insulator
layer between the first semiconductor layer and the second
semiconductor layer; and
the movable electrode and the fixed electrode are formed from the
second semiconductor layer with the first semiconductor layer and
the insulator layer underside the movable electrode and the fixed
electrode being removed.
19. A semiconductor sensor device of claim 1, further
comprising:
a connecting part connected to the fixed electrode and constructed
in the Rahmen structure.
20. A semiconductor sensor device of claim 1, further
comprising:
a connecting part connected to the fixed electrode and provided
only on the semiconductor substrate to reduce parasitic
capacitance.
21. A semiconductor sensor device comprising:
a semiconductor substrate;
a fixed electrode fixedly supported on the semiconductor substrate
at one end and forming a parasitic capacitor with the semiconductor
substrate; and
a movable electrode supported lovably on the semiconductor
substrate and forming a variable capacitor with the fixed
electrode,
wherein the fixed electrode has at least one groove which extends
from the semiconductor substrate, and
wherein said fixed electrode and said movable electrode are
constructed in Rahnen structure.
22. A semiconductor sensor device comprising:
a semiconductor substrate;
a fixed electrode fixedly supported on the semiconductor substrate
at one end and forming a parasific capacitor with the semiconductor
substrate; and
a movable electrode supported movably on the semiconductor
substrate and forming a variable capacitor with the fixed
electrode,
wherein the fixed electrode has at least one groove which extends
from the semiconductor substrate, and
wherein each of the fixed electrode and the movable electrode has a
plurality of through holes to provide a series connection of
rectangular frames which corresponds to Rahmen structure.
23. A semiconductor sensor device comprising:
a semiconductor substrate;
a movable unit supported on the semiconductor substrate, and having
a weight portion movable in response to a dynamical force exerted
thereon and a movable electrode formed integrally with the weight
portion and having a detection surface; and
a fixed electrode cantilevered on the semiconductor substrate and
having a detection surface facing the detection surface of the
movable electrode,
wherein each of the movable electrode and the fixed electrode
having a plurality of through holes arranged in a direction in
which the movable electrode and the fixed electrode extend, and
wherein said fixed electrode and said movable electrode are
constructed in Rahmen structure.
Description
BACKGROUND OF THE INVENTION
1. Field on the Invention
The present invention relates to a semiconductor type dynamical
quantity sensor device and, more particularly, to a differential
capacitor type semiconductor sensor device, which may be used as an
accelerometer device.
2. Related Art
In a conventional differential capacitor type semiconductor
accelerometer device 11, as shown in FIGS. 22 and 23, a weight
portion 15 and a comb-shaped movable electrode 16 are formed
integrally on a semiconductor layer of a semiconductor substrate
(Si) 19 to provide a movable unit 12. A pair of comb-shaped fixed
electrodes 17 and 18 are formed also on the semiconductor substrate
19 through an insulator layer (SiO.sub.2) 21 to face the movable
electrode 16. The movable electrode 16 and the fixed electrodes 17
and 18 are spaced apart and electrically insulated by a trench
formed on the semiconductor substrate 19 to provide capacitors
between detection surfaces thereof facing each other. The movable
unit 12 is supported at both ends thereof by the semiconductor
substrate 19 and movable in an axial direction of the movable unit
12 (in up-down direction in FIG. 22) in response to acceleration
exerted thereon to change the capacitance between the movable
electrode 16 and the fixed electrodes 17 and 18.
In this accelerometer device 11, the electrodes 16, 17 and 18 are
in plate shape and have respective self-weights. As a result, the
electrodes 16, 17 and 18 are likely to be broken by the respective
self-weights when a large acceleration is exerted in a direction
(up-down direction in FIG. 23) perpendicular to the direction of
acceleration to be detected (up-down direction in FIG. 22). If the
electrode width is narrowed to reduce the respective self-weights,
the strength of the electrodes against the torsion or twist force
will be lessened.
Further, in this accelerometer device 11, parasitic capacitors CP1,
CP2 and CP3 are formed in addition to capacitors CS1 and CS3
between the movable electrode 16 and the fixed electrodes 17 and 18
as shown in FIG. 24. Specifically, the capacitors CP1, CP2 and CP3
are formed between a connecting part 170 of the fixed electrode 17
and the substrate 19, between a connecting part 180 of the fixed
electrode 18 and the substrate 19 and between the movable electrode
16 and the substrate 19, respectively. The capacitors CS1 and CS3
are variable in response to the movement of the movable unit
12.
The capacitance changes of the capacitors CS1 and CS2 caused by the
acceleration may be detected by a switched capacitor circuit 10
connected to pads 28, 29 and 30 of the accelerometer device 11 as
shown in FIG. 25. Specifically, the switched capacitor circuit 10
comprises an amplifier AMP, a capacitor Cf and an on/off switch SW.
The circuit 10 is designed to operate differentially to produce an
output voltage Vo when carrier wave voltages CWV1 and CWV2 are
applied as shown in FIG. 26. The output voltage Vo is expressed as
follows.
As long as the capacitance of the parasitic capacitors CP1 and CP2
are equal to each other, the output voltage Vo varies solely in
accordance with changes in capacitance of the capacitors CS1 and
CS2. However, if the position of etching the substrate 19 varies as
shown by the dotted line in FIG. 24, the parasitic capacitor CP1
becomes larger than the parasitic capacitor CP2. This difference in
the parasitic capacitors CP1 and CP2 causes an offset voltage,
which is a deviation of the output voltage Vo from zero, even when
no acceleration is applied.
SUMMARY OF THE INVENTION
It is therefore an object of the present invention to provide a
semiconductor dynamical quantity sensor device, which is light in
weight and durable against accelerations in varying directions.
It is another object of the present invention to provide a
semiconductor dynamical quantity sensor device, which minimizes
parasitic capacitors to reduce an offset voltage.
According to the present invention, a semiconductor accelerometer
device is formed on an SOI substrate by micro-machining. A movable
unit has a weight portion and a comb-shaped movable electrode
formed integrally. A pair of comb-shaped fixed electrodes are
cantilevered and interleaved with the movable electrode to face the
movable electrode. When acceleration is exerted in the acceleration
detection direction in which the electrodes face, the weight
portion moves to change the capacitance between the movable
electrode and the fixed electrodes. A plurality of through holes is
provided in the electrodes so that the electrodes have Rahmen
structure, which is a series connection of rectangular frames. This
structure reduces the weight of each electrode while increasing the
strength against twist force. The electrodes are less likely from
breaking in response to acceleration exerted in a direction
perpendicular to the normal detection direction because of reduced
weight.
The electrodes are connected to a switched capacitor circuit, which
produces an output voltage corresponding to the capacitance changes
caused by the acceleration. To reduce offset of the output voltage,
connecting parts of the fixed electrodes to the circuit are also
formed in Rahmen structure, or formed only on an insulator of the
SOI substrate.
BRIEF DESCRIPTION OF THE DRAWINGS
Other objects, features and advantages of the present invention
will become more apparent from the following detailed description
made with reference to the accompanying drawings. In the
drawings:
FIG. 1 is a schematic plan view of a semiconductor accelerometer
device according to a first embodiment of the present
invention;
FIG. 2 is a sectional view of the accelerometer device taken along
line II--II in FIG. 1;
FIG. 3 is an enlarged plan view of a fixed electrode and a movable
electrode of the device shown in FIG. 1;
FIG. 4 is a schematic plan view of a pad, which is to be compared
with that of the device shown in FIG. 1;
FIG. 5 is a schematic plan view of a pad of the device shown in
FIG. 1;
FIGS. 6A and 6B are schematic sectional views of the fixed
electrode and the movable electrode of the device shown in FIG.
1;
FIGS. 7A and 7B are schematic sectional views of a fixed electrode
and a movable electrode to be compared with those shown in FIGS. 6A
and 6B;
FIG. 8 is a schematic perspective view of each finger portion of
the electrode used in the device shown in FIG. 1;
FIG. 9 is graph showing the relation between the weight of the
finger shown in FIG. 8 and the deformation of the same;
FIG. 10 is an enlarged plan view of a fixed electrode and a movable
electrode of a semiconductor accelerometer device according to a
modification of the first embodiment;
FIG. 11 is an enlarged plan view of a spring of a semiconductor
accelerometer device according to a modification of the first
embodiment;
FIG. 12 is an enlarged plan view of a pad of a semiconductor
accelerometer device according to a modification of the first
embodiment;
FIG. 13 is an enlarged plan view of a pad of another semiconductor
accelerometer device according to a modification of the first
embodiment;
FIG. 14 is a schematic sectional view of a fixed electrode and a
movable electrode of a semiconductor accelerometer device according
to a modification of the first embodiment;
FIG. 15 is a schematic sectional view of a fixed electrode and a
movable electrode to be compared with those shown in FIG. 14;
FIG. 16 is a schematic plan view showing electrostatic force which
exerts between the electrodes the device according to the first
embodiment;
FIG. 17 is a schematic plan view of a semiconductor accelerometer
device according to a modification of the first embodiment;
FIG. 18 is a schematic plan view of a semiconductor accelerometer
device according to a second embodiment of the present
invention;
FIG. 19 is a sectional view of the accelerometer device taken along
line XIX--XIX in FIG. 18;
FIG. 20 is a schematic plan view of a semiconductor accelerometer
device according to a third embodiment of the present
invention;
FIG. 21 is a sectional view of the accelerometer device taken along
line XXI--XXI in FIG. 20;
FIG. 22 is a schematic plan view of a conventional semiconductor
accelerometer device;
FIG. 23 is a sectional view of the conventional accelerometer
device taken along line XXIII--XXIII in FIG. 22;
FIG. 24 is a sectional view of the conventional accelerometer
device with parasitic capacitors and variable capacitors shown
therewith;
FIG. 25 is an electric circuit diagram of the conventional device
shown in FIG. 24; and
FIG. 26 is a timing diagram showing operation of the electric
circuit of the conventional device.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT
The present invention is described with reference to various
embodiments and modifications throughout which the same or similar
component parts are designated by the same or similar reference
numerals.
(First Embodiment)
Referring first to FIG. 1, a semiconductor accelerometer device 11
is formed on an SOI substrate by using the semiconductor
fabrication process.
A movable unit 12 has anchors 13, rectangle-shaped springs 14
integral with the respective anchors 13, a weight portion 15
integral with and provided between the springs 14, and a
comb-shaped movable electrode 16 integral from the weight portion
15. The movable electrode 16 has a plurality of parallel fingers
extending laterally from the weight portion 15 in opposite
directions. A pair of comb-shaped fixed electrodes 17 and 18 is
provided at both lateral sides of the weight portion 15. Each of
the fixed electrodes 17 and 18 has a plurality of fingers extending
laterally between the fingers of the movable electrode 16.
As shown in FIG. 2, the accelerometer device 11 is fabricated from
the SOI substrate, which comprises a first semiconductor layer (Si)
19, a second semiconductor layer (Si) 20 and an insulator layer
(SiO.sub.2) 21 as a support layer. The first semiconductor layer 19
and the insulator layer 21 are removed to expose the second
semiconductor layer 20 over the area where the movable unit 12 and
the fixed electrodes 17 and 18 are formed.
In fabricating the accelerometer device 11, aluminum (Al) is
vapor-deposited on the top surface of the SOI substrate at pad
portions 25 to 27 to form the electrode pad portions 28 to 30.
After polishing the bottom surface of the SOI substrate, plasma SiN
is accumulated. Then, the plasma SiN film is etched to form a
predetermined pattern.
Then, PIQ (polyimide) is pasted on the top surface of the SOI
substrate, and the PIQ film is etched in a predetermined pattern
which corresponds to the movable unit 12 and the fixed electrodes
17 and 18. A resist is pasted as a protective layer on the PIQ
film. The SOI substrate is etched deeply by, for instance, KOH
aqueous solution, while using the bottom side plasma SiN film as a
mask. In this deep etching, the insulator layer 21 functions as an
etching stopper, because the etching speed of the insulator layer
21 is slower than that of the Si semiconductor layer.
Next, after removing the exposed insulator layer 21 and plasma SiN
film by the HF aqueous solution, the resist covering the top
surface of the SOI substrate is removed. The second semiconductor
layer 20 is dry-etched to form holes therethrough, while using the
PIQ film as a mask. Thus, the movable unit 12 and the fixed
electrodes 17 and 18 are formed in the second semiconductor layer
20. Finally, the PIQ film on the top surface is removed by the
O.sub.2 ashing.
In the accelerometer device 11 as fabricated above, the both axial
ends of the movable unit 12 are supported on the insulator layer
21, and the fixed electrodes 17 and 18 are cantilevered on the
insulator layer 21.
A plurality of through holes 31 is formed in the movable unit 12
and the fixed electrodes 17 and 18, so that each of the movable
unit 12 and the fixed electrodes 17 and 18 is shaped in the Rahmen
structure (rigid frame structure). This structure is a series
connection of a plurality of rectangular frames. The through holes
31 are formed simultaneously with the holes (trench) which are
formed by dry-etching the second semiconductor layer 20 to form the
movable unit 12 and the fixed electrodes 17 and 18. Each finger of
the movable electrode 16 is positioned at the connection between
the adjacent two of the rectangular frames forming the weight
portion 16.
The movable unit 12 and the fixed electrodes 17 and 18 are sized to
satisfy the following relations (1) to (4), so that trench etching
may be performed accurately. In the following relations, width
(thickness of frame wall) W1 to W4 and width (spacing interval
between frame walls) D1 to D5 are defined as shown in FIG. 3.
Specifically, W1 to W4 and D1 to D5 are defined as follows.
"W1": axial width of each laterally extending wall of the fingers
of the electrodes 16, 17 and 18;
"W2": axial width of each laterally extending wall of the weight
portion 15 existing at the connection with each finger of the
movable electrode 16, and "W2" also equals the total width of each
rectangular frame of electrodes 16, 17 and 18 (see equation (4)
below);
"W3": lateral width of each axially extending wall of the weight
portion 15;
"W4": lateral width of each axially extending wall of the fingers
of the electrodes 16, 17 and 18;
"D1": axial width of each through hole 31 in the fingers of the
electrodes 16, 17 and 18;
"D2": axial width between facing surfaces of the adjacent fingers
of the electrodes 16, 17 and 18, the facing surfaces being for
detecting capacitance changes therebetween;
"D3": lateral width of each through hole 31 of the weight portion
15;
"D4": lateral width between the weight portion 15 and each free end
of the fixed electrodes 17 and 18; and
"D5": axial width between facing surfaces of the adjacent fingers
of the electrodes 16, 17 and 18, the facing surfaces being not for
detecting capacitance changes therebetween.
(1) W1=W3=W4
(2) D1=D2=D3
(3) D4=D5
(4) W2=W1.times.2+D1
As shown in FIG. 3, the fixed electrodes 17 and 18 have a plurality
of protrusions 32 at the respective detection surfaces which face
the detection surfaces of the movable electrode 16. The protrusions
32 are only on the surfaces to be used to detect capacitance change
in response to the movement of the movable electrode 16 relative to
the fixed electrodes 17 and 18. As the spacing interval between the
movable electrode 16 and the fixed electrodes 17 and 18 varies in
response to the movement of the weight portion 15 when subjected to
the acceleration, the movable electrode 16 is likely to stick to
the fixed electrodes 17 and 18 because of the external
electrostatic force. This sticking disables the detection of
capacitance changes which correspond to the applied acceleration.
The protrusions 32, however, restrict the sticking of the movable
electrode 15 to the fixed electrodes 17 and 18. The protrusions 32
are formed preferably only on the parts where no through holes 31
are formed, that is, only on the connection parts (width W4) of the
rectangular frame structures which constitute the fingers of the
fixed electrodes 17 and 18.
Similarly, as shown in FIG. 1, a plurality of protrusions 33 is
formed on the inside surface of the spring 14 to restrict sticking
between a pair of the laterally extending parts because of the
external electrostatic force. The protrusions 33 are formed at the
connecting position (width W3) between the rectangular frames in
the anchor 13.
The pad portions 25 to 27 are separated physically and electrically
by a pair of grooves 34 from the surrounding portions which is the
second semiconductor layer 20, so that the electric pads 28 to 30
connected to the corresponding electrodes 16, 17 and 18 through
connecting parts 170, 180 and the like are electrically connected
to an external detection circuit such as the switched capacitor
circuit 10 shown in FIG. 25. If only one groove 34 is formed as
shown in FIG. 4, the pad portions 25 to 27 are likely to short to
the surrounding portions in the event that a conductive foreign
matter such as a conductive dust bridges the groove 34. This
shorting problem may be eliminated by widening the groove 34.
However, this groove width becomes different from the other groove
width, resulting in complication of fabricating process and
reduction in the accuracy in final product size. Therefore, in this
embodiment, two grooves 34 are formed to provide a bank wall 35
therebetween as shown in FIG. 5 to reduce the possibility of
shorting between the pad portions 25 to 27 and the surrounding
portions. Thus, each groove 34 may be sized to the same width as
the other grooves (trench or hole).
Each of the electrodes 16, 17 and 18 are tapered from the middle
part toward the bottom side as indicated at 36 in FIG. 6A. That is,
a predetermined capacitance is provided between the upper halves of
the detection surfaces of the electrodes 16, 17 and 18 facing each
other as shown by dotted lines. Even if notches 16a, 17a and 18a
are produced on the tapered surfaces as shown in FIG. 6B in the
course of forming electrodes, the predetermined capacitance (dotted
lines) is maintained. This is because the notches 16a, 17a and 18a
will occur on the tapered surfaces only. On the contrary, if the
electrodes 16, 17 and 18 are not tapered as shown in FIG. 7A, the
capacitance (dotted line) is likely to decrease because of the
notches 16a, 17a and 18a occurring at the lower halves of the
electrodes 16, 17 and 18 as shown in FIG. 7B. Those notches will
vary from wafer to wafer and from chip to chip, causing
irregularity in capacitance among the final products.
The above semiconductor accelerometer 11 is sized preferably as
follows:
(1) Width of anchor 13 and weight portion 15=10-200 .mu.m;
(2) Length of electrodes 15, 16 and 17=100-500 .mu.m;
(3) Width of spring 14=2-10 .mu.m;
(4) Length of spring 14=100-500 .mu.m; and
(5) Spacing between electrode 16 and electrodes 17 and 18=2-4
.mu.m.
In operation, when the acceleration is exerted on the movable unit
12 in the acceleration detection direction (X in FIG. 1), that is,
in the axial direction in which the movable electrode 16 faces the
fixed electrodes 17 and 18, one of the spacing interval between the
detection surfaces of movable electrode 16 and the fixed electrodes
17 and 18 increases, and the other of the spacing interval between
detection surfaces of the movable electrode 16 and the fixed
electrodes 17 and 18 decreases. As those detection surfaces form
capacitors, the respective capacitance change in response to the
acceleration. Those changes are detected by the switched capacitor
circuit 10 shown in FIG. 25, for instance.
In the event that acceleration is exerted on the moving unit 12 in
a direction (up-down direction in FIG. 2) perpendicular to the
normal detection direction (X), the moving unit 12 and the fingers
of the fixed electrodes 17 and 18 are less likely to break because
all of the moving unit 12 and the fixed electrodes 17 and 18 are
constructed in light weight by the use of Rahmen structure.
More specifically, in the event that a rod shown in FIG. 8 is
deformed, the deformation of the rod and the maximum stress which
exerts on the rod is expressed as follows.
Deformation=(acceleration.times.weight).div.(spring constant in
deformation direction)
Maximum stress=2.times.(Young's modulus).times.(T or W).times.(rod
deformation).div.L.sup.2
As a result, as shown in FIG. 9, the deformation of rod and the
maximum stress increase as the weight of the rod increases. In the
case of the movable unit 12 fixedly supported at both ends and the
fixed electrodes 17 and 18 fixedly supported only at one end, the
influence of the acceleration exerted in the direction
perpendicular to the normal detection direction is reduced more as
the weight is lighter. Therefore, Rahmen structure is effective to
reduce the weight of the movable unit 12 and the fixed electrodes
17 and 18 for less deformation and less stress without lessening
the strength against the twist force.
The widths of the rectangular frame walls and the spacing interval
between the frame walls of the moving unit 12 and the fixed
electrodes 17 and 18 are sized uniform as much as possible.
Therefore, variations in size of the component parts can be reduced
to a minimum, and the final products can have the uniform
quality.
As the anchor 13 and the weight portion 15 are constructed to have
the same Rahmen structure as in the electrodes 16, 17 and 18, the
finished size after etching can be maintained uniformly to provide
finished products with uniform quality. In addition, as the fingers
of the movable electrode 16 are connected to the connection
position of the rectangular frames in the weight portion 15, that
is, connected to the most rigid part of the weight portion 15, the
movable electrode 16 can be maintained resistive to the
acceleration exerted in the direction perpendicular to the normal
detection direction.
(Modification)
The electrodes 16, 17 and 18 may be formed with reinforcing
portions 16b, 17b and 18b in arcuate shape at the respective roots
of the fingers as shown in FIG. 10. Those reinforcing portions 16b,
17b and 18b strengthen the connection of the fingers to restrict
breakage of the fingers even when stress concentrates at the root
portions in response to acceleration exerted in the direction
perpendicular to the normal detection direction.
Similarly, the spring 14 may be connected to the anchor 13 and the
weight portion 15 through arcuate reinforcing portions 14a as shown
in FIG. 11. Those reinforcing portions 14a restrict the spring 14
from breaking even when stress concentrates at the connection
portion. Further, the ends of the spring 14 may be formed in an
arcuate shape to restrict the spring 14 from breaking when stress
concentrates at the ends because of resilient flexing of the spring
14.
The bank wall 35 surrounded by the grooves 34 formed around the pad
portions 25 to 27 may be cut dynamically and electrically at one
location as shown in FIG. 12 or at a plurality of locations as
shown in FIG. 13. Those cuts can greatly reduce possibility of
electrical shorting between the pad portions 25 to 27 and the
surrounding portions (second semiconductor layer 20), even when the
bank wall 35 is connected to the surrounding portion through a
conductive dust and also to the pad portions 25 to 27 through
another conductive dust.
Still further, the electrodes 16, 17 and 18 may be tapered at both
top side and bottom side as shown in FIG. 14. Alternatively, the
corners of the electrodes 16, 17 and 18 may be rounded as shown in
FIG. 15. The rounded corners will reduce influence caused by
notches and restrict concentration of stress even when acceleration
is exerted on the electrodes 16, 17 and 18.
In the first embodiment, the electrostatic force exerts between the
movable electrode 16 and the fixed electrodes 17 and 18 in opposite
directions as shown by arrows in FIG. 16. This electrostatic force
results in the moment which tends to rotate the movable unit 12 in
the clockwise direction. Thus, the spacing interval between the
capacitance detection surfaces between the movable electrode 16 and
the fixed electrodes 17 and 18 are likely to deviate from the
original spacing interval, resulting in lessening the accuracy of
acceleration detection. It is therefore preferred to arrange four
fixed electrodes 171, 172, 173 and 174 as shown in FIG. 17, so that
the electrostatic force exerted between the movable electrode 16
and the fixed electrodes 171 and 181 balances with the
electrostatic force exerted between the movable electrode 16 and
the fixed electrodes 172 and 182. Thus, the moment which exerts to
rotate the movable unit 12 is restricted.
The material used for the structural body of the accelerometer
device may be a single silicon, poly silicon or metal. Further,
nonconductive material such as ceramics, glass, crystal or resin
may be used for the structural body as long as a conductive
material is vapor-deposited thereon. In this instance, the SOI
structure need not be provided as long as the material for the
structural body has an insulating property.
(Second Embodiment)
In this embodiment, as shown in FIGS. 18 and 19, not only the
fingers of the movable electrode 16 and the fixed electrodes 17 and
18 are constructed in Rahmen structure, but also connecting
portions 170 and 180 connecting the fixed electrodes 17 and 18 to
the pads 29 and 30 are also constructed in Rahmen structure by a
plurality of through holes 31.
According to this embodiment, the capacitance of the parasitic
capacitors CP1, CP2 and CP3 which occur as shown in FIG. 24 can be
reduced to a smaller value than in the conventional device (FIG.
22). Therefore, when this accelerometer device 11 is connected to
the switched capacitor circuit 10 as shown in FIG. 25, offset of
the output voltage Vo produced from the switched capacitor circuit
10 is reduced even when etching of the bottom side of the
semiconductor layer 21 varies in the fabrication process.
(Third Embodiment)
In this embodiment, as shown in FIGS. 20 and 21, the first
semiconductor layer 19 and the insulator layer 21 have respective
innermost ends 19a and 21a at a position below the fixed electrodes
17 and 18. That is the connecting portions 170 and 180 are formed
only on the insulator layer 21. As a result, the capacitance of the
parasitic capacitors CP1, CP2 and CP3 which occur as shown in FIG.
24 can be reduced further than in the second embodiment.
When this accelerometer device 11 is connected to the switched
capacitor device as shown in FIG. 25, the detected acceleration is
represented by the output voltage Vo which is expressed as
follows:
As (CP1-CP2).times.CP3 is reduced also because of reduction in the
capacitance of the parasitic capacitors, the resulting offset of
the output voltage Vo produced from the switched capacitor circuit
10 is reduced even when etching of the bottom side of the
semiconductor layer 21 varies in the fabrication process.
It is to be noted that the above modifications of the first
embodiment may also be applied to the second and third
embodiments.
Further, it is to be noted that the present invention may also be
applied to other dynamical quantity sensor devices such as a yaw
rate sensor and an angular velocity sensor. Further, the present
invention may be applied to a capacitive type semiconductor
pressure sensor, which has a diaphragm as a sensing structural body
and uses the diaphragm as a movable electrode. Still further, the
present invention may be applied to a contact type sensor, which
detects on and off between a movable electrode and a fixed
electrode.
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